A conventional metal sheet processing apparatus such as a press brake comprises left and right side frames and upper and lower tables supported by the side frames. The upper and lower tables are disposed one above another and face each other. One of the upper and lower tables serves as a vertically movable ram. An upper die is removably attached to the bottom of the upper table, and a lower die is removably attached to the top of the lower table.
The upper and lower dies are replaced with others depending on bending conditions of metal sheets.
The upper and lower dies are usually manually removed and replaced. Since the upper and lower dies are generally long (there are divided dies comprising divided short parts), it is quite troublesome to remove and attach the upper and lower dies from and to the upper and lower tables.
Accordingly, techniques for automatically attaching, detaching, and replacing the upper and lower dies have been developed. For example, there are such related arts as Japanese Examined Patent Publication Nos. 55-45288 and 57-37408 and Japanese Examined Utility Model Publication No. 63-21932.
In this way, various techniques have been developed for automatically attaching, detaching, and replacing upper and lower dies to and from upper and lower tables of a press brake. One of such techniques arranges, beside the press brake, a storage unit for storing upper and lower dies. This technique takes a large space. Another technique defines a storage section in the frame of a press brake. This technique must have a structure of preventing a back gauge unit from interfering with upper and lower dies when replacing the dies with others, and does not utilize the back gauge unit for replacing the dies.
An object of the invention is to provide a plate material processing apparatus that utilizes a back gauge unit not only for positioning a metal sheet in longitudinal directions but also for replacing upper and lower dies. Namely, the invention utilizes the back gauge unit both for replacing upper and lower dies and positioning a metal sheet.